1. Field of the Invention
The present invention relates to a device to heat and humidify gas, especially respiratory gas. The device comprises a fluid reservoir, a humidification chamber with gas inlet and outlet, and a device to move the fluid through the gas. Furthermore, the invention relates to such a process.
Such a device and process should be used especially for the artificial ventilation of a patient. With respect to the here presented invention, “ventilation” comprises each type of respiratory therapy, and “patient” comprises as well human as well as animal patients.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Traditionally such a type of patient ventilation is performed using a device with a respiratory tubing, a device to generate the gas flow directed to the patient (“gas flow generator”), and a respiratory gas humidifier. Traditionally the respiratory gas is taken from a reservoir and directed to the gas flow generator, and the gas flow generator is equipped with a gas outlet connecting to the respiratory tubing.
The main feature of such a gas flow generator is that it is capable to control pressure and/or volume and/or flow of the respiratory gas directed to the patient for instance by means of valves or a bellow. The gas flow generator may be a separate device, or it may be integrated into another device.
Usually, the respiratory gas is a mixture of air and oxygen, but other special gas mixtures may be used as well.
Usually, the fluid within the fluid reservoir which is used to humidify the respiratory gas (air; mixture of air and oxygen; else) is water. Within the scope of the here presented invention one could imagine to use water as well as other fluids, or a mixture of different fluids, with or without added drugs.
Most artificially ventilated patients are ventilated via an endotracheal tube. This tube connects to the distal end of a respiratory tubing leading the respiratory gas from the respiratory gas flow generator to the patient. The tube may well simultaneously connect to several other respiratory tubings.
Usually, the respiratory gas delivered by the respiratory gas flow generator is controlled by several parameters in order to adapt the respiratory gas flow to the patient's individual needs. The set parameters are automatically controlled by the respiratory gas flow generator or may be adjusted according to requirements. Depending on the model of respiratory gas flow generator used and its control settings the instantaneous respiratory gas flow may vary widely during a single respiratory cycle which when using so far known ventilators may lead to serious problems.
The endotracheal tube bypasses nose, pharynx and larynx thus eliminating their normal function of heating and humidification of the respiratory gas. While using a face mask for the application of continuous positive airway pressure (CPAP), nose and pharynx/larynx are functionally maintained. During respiratory therapy, however, the significantly higher and often continuous gas flow with reference to normal breathing—especially when using cold and dry respiratory gas—often leads to adverse effects, i.e. irritation, inflammation, dryness and incrustation of the upper airways.
In order to overcome those problems, during respiratory therapy the respiratory gas taken from a reservoir (i.e. pressurized gas from a bottle or wall outlet), or from the environment via fan or bellow etc., which usually is quite dry is artificially humidified and heated.
The aim is to emulate the natural conditions, that is to heat the respiratory gas to body temperature and to humidify it to nearly full saturation. The aim is to reach a saturation of 95 to 100% relative humidity.
That task is quite difficult to perform especially under circumstances as described above where the instantaneous respiratory gas flow is widely varying as during spontaneous breathing or artificial ventilation.
Similar difficulties exist in other medical areas than respiratory therapy, i.e. laparoscopy, and there devices similar to the here presented invention may be used. During laparoscopy, for purposes of expansion a gas (frequently used gas: carbon dioxide) is insufflated into a body cavity (e.g. abdomen). In that application and similar ones heating and humidification of the gas to nearly saturation is capable to prevent the quite frequent adverse effects of mucosal irritation and drying and cooling.
It is important to note that also in laparoscopy the gas flow shows remarkably high variations in time, since the insertion of instruments into or their removal from body openings requires a fast gas flow control in order to maintain a constant pressure within the body cavity.
Principally there are several techniques and processes known to heat and humidify gas to preset values in the applications mentioned above. The following describes some of those processes and the corresponding devices.
Pass-Over Humidifier (e.g. DE 38 30 314)
This known device uses a reservoir filled with heated water. The respiratory gas is conducted along the water's surface thus heating and at the same time humidifying the gas. The water at the surface will cool down due to evaporation and is rather slowly replaced by warmer water mounting from beneath.
The area of the water/gas interface is limited due to the limited space available in the practical respiratory setting. In conjunction with the cooling of the surface water and its but slow replacement with warmer water (see above) resulting in a slow energy transfer to the water surface such a humidifier will deliver gas with a temperature highly dependent on gas flow, i.e. a varying gas-fluid temperature difference. This is why with that humidifier design the water's temperature is set according to the temporal average gas flow. In conventional devices this temperature is between about 40 and 80° C. Hence the instantaneous temperature and humidity of a heavily varying gas flow which is commonly seen in respiratory therapy (see above) is either too high or too low. A theoretical but technically impractical solution would be a very fast water temperature control.
Membrane-Type Humidifier (e.g. DE 43 03 645)
With such a device gas is directed over the surface of a structured body protruding from the heated fluid. The structured body is sucking from the reservoir the amount of fluid needed, e.g. by capillary forces. Only the amount of fluid just evaporated is replaced by fresh heated fluid. The most significant disadvantage of that design is that there arise similar problems as with the pass-over humidifier, since there is generated evaporation coldness which is not compensated for as fast as needed due to the rather slow energy transfer by the heated water rather slowly replacing the evaporated water, resulting in the inability to humidify or heat a highly variable gas flow impossible to constant values. The life-time of most of the structured bodies on the market is limited, and most of them are not fully autoclavable which is disadvantageous with respect to medical applications.
Fibre-Type Humidifier (e.g. DE 197 27 884)
Partially permeable hollow fibers (e.g. from PTFE) are bundled, and the gas to be heated and humidified is directed through their luminae. The outer surface of the fibers is in contact with the fluid needed for humidification. The disadvantage of that design is the fibers' limited life-time and mechanical as well as thermal durability. Moreover, the fibers' unsuitably high thermal resistance unduly restricts the heat transfer needed to compensate for evaporation coldness. Thus especially with high gas flow the heating of the gas is insufficient, which in turn leads to insufficient gas humidification. From theory increasing the water's temperature might compensate for those limitations. In case of a heavily varying gas flow, however, even forced heating of the fibers will not lead to constant humidification due to technical limitations of controlling the instantaneous fibre temperature as fast as required.
High-Temperature Humidifier (e.g. DE 43 12 793)
With such devices small quantities of fluid are evaporated at temperatures of about 80° C. to 130° C. and mixed with the gas flow, thus providing both the energy to heat the gas and the humidity as required. Main disadvantage of those devices is the high technical complexity needed which is paralleled by an increased technical risk especially with respect to high pressure and heat. Another disadvantage is that for technical reasons the control of the evaporation lags behind the demands. With heavily varying gas flow this will result in inconstant heating and humidification.
Bubble Through Humidifier (e.g. DE 37 30 551)
With those devices gas is bubbling through a heated fluid, resulting in heating and humidifying of the gas. The main disadvantage of that design is its high gas flow resistance which numerically is at least the pressure difference resulting from the fluid surface to the level where the gas is entering the fluid. Especially in spontaneously breathing patients a high gas flow resistance is disadvantageous.
Ultrasound-Type Nebulizer (e.g. DE 197 26 110)
Those devices use ultrasound to induce fluid vibrations resulting in the generation of tiny droplets which enter the gas flow. Main disadvantage of that design is that the “humidification” doesn't result in molecular fluid within the gas but in substantially larger fluid particles (generation of an aerosol). In contrast to molecular fluid, those larger particles have the potential to transport pathogens to the patient. There is also the risk that—especially with intermittent or varying gas flow—the amount of humidity is too high or too low.
Pressure-Type Nebulizer (e.g. DE 28 34 622)
Those devices nebulize a fluid resulting in the formation of tiny droplets, not molecular fluid. Thus those devices inherit the same disadvantages as ultrasound-type nebulizers (see above).
Heat and Moisture Exchanger (“HME” e.g. DE 94 17 169), Filter Pads, etc.
With heat and moisture exchangers (“artificial noses”) the gas is directed over a very large wet surface which results in saturation of the gas with humidity. The “artificial nose” extracts the heat and humidity needed from the patient's expiratory gas. Filter pads e.g. from air conditioning technique get the heat and humidity needed from a water bath or similar device. While heating and humidifying the gas filter pads filtrate it from particles.
In all those devices it is of disadvantage that the evaporation coldness results in a gas flow dependent decrease in gas temperature. Thus with varying gas flow it is impossible to provide constant gas temperature and humidity. The amount of particles adhering to the filter will increase with time resulting in an increase in gas flow resistance which is highly undesirable in the medical setting. Since by design HME have to be placed in the patient's inspiratory as well as expiratory gas stream they increase the dead space with the result that the patient will inspire more or less his expiratory gas.
Booster Systems (e.g. DE 44 32 907)
With those systems it is tried to compensate for the insufficient efficiency of an “artificial nose” (HME) by means of adding both fluid and heat which requires a technically demanding control circuitry. By design, a heavily varying gas flow will result in inconstant gas temperature and humidity, since even the best control circuit is incapable to compensate for the evaporation coldness without significant time lag. Of disadvantage are also the system's increased dead space (see above), its large dimension and weight, and other features not discussed here.
A Combination of the Above Mentioned Systems (e.g. DE 296 12 115)
With this combined design first the gas is overheated and humidified. Then in a following second step the gas is cooled down to target temperature by means of metal lamellas or equivalent. During that step any humidity above saturation will form condensate dripping from the metal lamellas. The condensate is recirculated to the humidifier. With this process it is of disadvantage that at first more energy in form of humidity and temperature is added to the gas than needed for ventilation.
This is not only unfavorable from an energetic point of view but there is also the risk that any malfunction of the cooling system will result in a substantial damage to the patient.
Ambient Air Humidifier with a Stack of Rotating Plates (e.g. DE 37 35 219)
Those systems inherit a stack of rotating plates which during a part of each rotation dip into water thus becoming wet. A fan drives the gas along those stacks. The idea is that so the gas will be both cleared from any particles, and humidified. Those systems need a non-volatile additive in the fluid to reduce the fluid's surface tension thus allowing for a sufficient wetting of the stack of plates. Such, or similar, devices are intended for use on climatization of living spaces. They lack any possibility to heat the gas to a preset value. The practically limited mechanical dimension of those systems as well as their rotation frequency render them unsuitable for constant humidification of a varying gas flow, or to saturate it with humidity.
Thus to summarize, the state-of-the-art gas heating and humidifying systems are unsuitable for a satisfying controlled heating and humidification of a heavily varying or intermittent gas flow. Under those conditions, the so far known systems produce a gas with heavily varying temperature and humidity.
Another disadvantage of the known systems is that they impair—some of them to a very high degree—the precision of measurements and regulatory control loops highly desirable in respiratory therapy, or they hinder them totally. For instance, in ventilatory support applications some methods and sensors require the direct coupling of the respiratory gas flow generator or a sensor to the patient in order to deliver the respiratory gas in a preset and constant quality and quantity, for instance a precise volume flow.
Disadvantages of the already known humidifier devices placed between the respiratory gas flow generator and the patient are that they add an extra compressible volume—sometimes of extraordinary magnitude—to the respiratory circuit.
Another disadvantage of some of the already known humidifier devices is the positive pressure gradient between gas inlet and outlet. Since the pressure of the respiratory gas given to the patient (level of respiration gas) is only marginally higher than the pressure of the ambient air (pressure difference usually max. 0.1 bar) the pressure within the gas flow generator is not the same as that directly at the patient. In consequence there is the risk of malfunction and imprecise pressure control.